1. Field of the Invention
The present invention relates to a battery charger and charging method for charging batteries or the like, and more particularly to prevent undesirable operation thereof.
2. Description of the Prior Art
The charging of batteries such as nickel-cadmium cells, involves a problem relating to overcharge which may damage the batteries. A battery charger of prior art cuts-off a charging current by detecting a charge completion by measuring the voltage of the battery being charged.
FIG. 1 shows a battery charger of the prior art described in U.S. Pat. No. 4,006,397. Current from a charge current source 90 is fed to a battery 93 through a fast charge current switch 92 such as relay. The fast charge current switch 92 is in turn controlled by an OR logic gate 96 which is in turn coupled to a temperature threshold detector 95 and a voltage threshold detector 94. The voltage threshold detector 94 senses the voltage of the battery 93 and feeds a corresponding signal back to the OR logic circuit 96. The logic circuit 96 compares the voltage received from the voltage threshold detector 94 to a reference voltage. When the received voltage from the voltage threshold detector 94 exceeds the reference voltage a signal is passed to the fast charge current switch 92 to disconnect the fast charge current path.
Following is an example of the detection for charge completion of the battery in another prior art. As shown in FIG. 9, the voltage of the battery varies with elapsed time from start of the charging. In case of continuing the charging even after completing the charging (shown by a time t.sub.1), the voltage of the battery slightly decreases. The decrease in voltage -.DELTA.V is porportional to the number of cells in the battery. Therefore, the completion of the charging can be known by detecting the voltage decrease -.DELTA.V.
However, the prior art has not been able to use a switching regulator as a charge current source which enables use of a small transformer. This is because the switching regulator causes a noise on the detected voltage of the battery. The noise may make erroneous detection of charge completion, for example, erroneous charge termination in spite of no completion of charge or erroneous charge continuation in spite of completion of charge.